scholarly journals Pacific Climate Change and ENSO Activity in the Mid-Holocene

2009 ◽  
Vol 22 (4) ◽  
pp. 923-939 ◽  
Author(s):  
J. C. H. Chiang ◽  
Y. Fang ◽  
P. Chang
Keyword(s):  
Annual Cycle ◽  
Climate Changes ◽  
Southern Oscillation ◽  
Asian Summer Monsoon ◽  
Coupled Model ◽  
Tropical Pacific ◽  
Orbital Forcing ◽  
Frequency Entrainment ◽  
Asian Summer ◽  
The Tropical Pacific

Abstract The authors argue that a reduction to the stochastic forcing of the El Niño–Southern Oscillation (ENSO) wrought by Pacific-wide climate changes in response to mid-Holocene (6000 BP) orbital forcing is a viable hypothesis for the observed reduction of ENSO activity during that time. This conclusion is based on comprehensive analysis of an intermediate coupled model that achieves significant reduction to ENSO variance in response to mid-Holocene orbital forcing. The model’s excellent simulation of the tropical Pacific interannual variability lends credibility to the results. Idealized simulations demonstrate that the mid-Holocene influence is communicated to the tropical Pacific largely via climate changes outside of the tropical Pacific, rather than from insolation changes directly on the tropical Pacific. This is particularly true for changes to the ENSO, but also with changes to the cold tongue annual cycle. Previously proposed mechanisms for teleconnected mid-Holocene ENSO changes, including forcing of ENSO by a strengthened Asian summer monsoon and an increase in the annual cycle forcing on the tropical Pacific leading to a reduction in ENSO activity by frequency entrainment, do not appear to occur in these simulations. Rather, the authors show that the modeled mid-Holocene climate exhibits a pronounced reduction in Pacific meridional mode activity that has been recently shown to be a forcing on ENSO, though the reasons for this reduction are still to be explained. The contrasting nature of the results compared to previous studies highlights the effect of the prevailing ENSO paradigm on this problem. By showing that an externally forced ENSO model is equally capable of explaining mid-Holocene ENSO reduction as its nonlinear, weakly chaotic counterpart, it is demonstrated that the mid-Holocene ENSO data point cannot yet discriminate between these two paradigms of ENSO.

scholarly journals Biophysical Feedbacks in the Tropical Pacific

2005 ◽  
Vol 18 (1) ◽  
pp. 58-70 ◽  
Author(s):  
Ben Marzeion ◽  
Axel Timmermann ◽  
Ragu Murtugudde ◽  
Fei-Fei Jin
Keyword(s):  
Mixed Layer ◽  
Annual Cycle ◽  
Southern Oscillation ◽  
Heat Budget ◽  
Tropical Pacific ◽  
Shortwave Radiation ◽  
Bjerknes Feedback ◽  
Mean State ◽  
Chlorophyll Concentrations ◽  
The Tropical Pacific

Abstract This study explores the influence of phytoplankton on the tropical Pacific heat budget. A hybrid coupled model for the tropical Pacific that is based on a primitive equation reduced-gravity multilayer ocean model, a dynamic ocean mixed layer, an atmospheric mixed layer, and a statistical atmosphere is used. The statistical atmosphere relates deviations of the sea surface temperature from its mean to wind stress anomalies and allows for the rectification of the annual cycle and the El Niño–Southern Oscillation (ENSO) phenomenon through the positive Bjerknes feedback. Furthermore, a nine-component ecosystem model is coupled to the physical variables of the ocean. The simulated chlorophyll concentrations can feed back onto the ocean heat budget by their optical properties, which modify solar light absorption in the surface layers. It is shown that both the surface layer concentration as well as the vertical profile of chlorophyll have a significant effect on the simulated mean state, the tropical annual cycle, and ENSO. This study supports a previously suggested hypothesis (Timmermann and Jin) that predicts an influence of phytoplankton concentration of the tropical Pacific climate mean state and its variability. The bioclimate feedback diagnosed here works as follows: Maxima in the subsurface chlorophyll concentrations lead to an enhanced subsurface warming due to the absorption of photosynthetically available shortwave radiation. This warming triggers a deepening of the mixed layer in the eastern equatorial Pacific and eventually a reduction of the surface ocean currents (Murtugudde et al.). The weakened south-equatorial current generates an eastern Pacific surface warming, which is strongly enhanced by the Bjerknes feedback. Because of the deepening of the mixed layer, the strength of the simulated annual cycle is also diminished. This in turn leads to an increase in ENSO variability.

The Annual Cycle, Intraseasonal Oscillations, and Roadblock to Seasonal Predictability of the Asian Summer Monsoon

2006 ◽  
Vol 19 (20) ◽  
pp. 5078-5099 ◽  
Author(s):  
B. N. Goswami ◽  
Guoxiong Wu ◽  
T. Yasunari
Keyword(s):  
Empirical Evidence ◽  
Summer Monsoon ◽  
Annual Cycle ◽  
Asian Monsoon ◽  
Southern Oscillation ◽  
Asian Summer Monsoon ◽  
Internal Variability ◽  
Systematic Bias ◽  
Intraseasonal Oscillations ◽  
Asian Summer

Abstract Factors responsible for limited predictability of the Asian summer monsoon (ASM) are investigated within a conceptual framework for predictability. Predictability of the seasonal mean depends on the interannual variability (IAV) of the monsoon annual cycle (MAC) and is determined by relative contribution of the predictable “external” component of IAV compared to the unpredictable “internal” IAV. Contributions of slow processes such as those involving air–sea interactions associated with the El Niño–Southern Oscillation (ENSO) or local warm ocean–atmosphere interactions in generating IAV of the MAC are reviewed. Empirical evidence that these air–sea interactions modulate the MAC is presented. Estimates of internal IAV have been made from observations as well as atmospheric model simulations. In contrast to a large part of the Tropics where the summer climate is predictable, with the internal variability being much smaller than the external one, the limited predictability of the Asian monsoon appears to be due to the fact that the contribution from the external IAV over the region is relatively weak and comparable to that from internal IAV. Cause for large internal IAV over the ASM region is investigated, and it is proposed that the internal IAV of the MAC is primarily due to interaction between the MAC and the summer intraseasonal oscillations (ISOs). Two mechanisms through which ISOs lead to internal IAV of the MAC are unraveled. The seasonal bias of the ISO anomalies can influence the seasonal mean if the spatial structure of the ISO has significant projection on that of the seasonal mean and if frequency of occurrence of positive and negative phases is unequal. Evidence supporting this is presented. In addition, it is demonstrated that the chaotic summer ISOs modulated by the annually varying forcing associated with the “slow annual cycle” can lead to IAV of the seasonal mean. Empirical evidence that IAV of ISO activity is related to IAV of the seasonal mean or MAC is also presented. Thus, the Asian monsoon would remain a difficult system to predict. To exploit the predictable signal, however, it is imperative that systematic bias of the models is improved and the space–time structure of the summer ISOs is simulated accurately.

scholarly journals Impact of westerly wind burst on El Niño-sourthern oscillation

2020 ◽  
Author(s):  
◽  
Mohammad Alam
Keyword(s):  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Coupled Model ◽  
Vital Role ◽  
Tropical Pacific ◽  
Westerly Wind ◽  
Central Pacific ◽  
Enso Events ◽  
The Tropical Pacific

Westerly wind bursts (WWBs), usually occurring in the tropical Pacific region, play a vital role in El Niño–Southern Oscillation (ENSO). In this study, we use a hybrid coupled model (HCM) for the tropical Pacific Ocean-atmosphere system to investigate WWBs impact on ENSO. To achieve this goal, two experiments are performed: (a) first, the standard version of the HCM is integrated for years without prescribed WWBs events; and (b) second, the WWBs are added into the HCM (HCM-WWBs). Results show that HCM-WWBs can generate not only more realistic climatology of sea surface temperature (SST) in both spatial structure and temporal amplitudes, but also better ENSO features, than the HCM. In particular, the HCM-WWBs can capture the central Pacific (CP) ENSO events, which is absent in original HCM. Furthermore, the possible physical mechanisms responsible for these improvements by WWBs are discussed.

scholarly journals The Effect of Orbital Forcing on the Mean Climate and Variability of the Tropical Pacific

2007 ◽  
Vol 20 (16) ◽  
pp. 4147-4159 ◽  
Author(s):  
A. Timmermann ◽  
S. J. Lorenz ◽  
S-I. An ◽  
A. Clement ◽  
S-P. Xie
Keyword(s):  
Annual Cycle ◽  
General Circulation ◽  
Model Simulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Orbital Forcing ◽  
Frequency Entrainment ◽  
Eastern Equatorial Pacific ◽  
The Mean ◽  
Mean Climate

Abstract Using a coupled general circulation model, the responses of the climate mean state, the annual cycle, and the El Niño–Southern Oscillation (ENSO) phenomenon to orbital changes are studied. The authors analyze a 1650-yr-long simulation with accelerated orbital forcing, representing the period from 142 000 yr b.p. (before present) to 22 900 yr a.p. (after present). The model simulation does not include the time-varying boundary conditions due to ice sheet and greenhouse gas forcing. Owing to the mean seasonal cycle of cloudiness in the off-equatorial regions, an annual mean precessional signal of temperatures is generated outside the equator. The resulting meridional SST gradient in the eastern equatorial Pacific modulates the annual mean meridional asymmetry and hence the strength of the equatorial annual cycle. In turn, changes of the equatorial annual cycle trigger abrupt changes of ENSO variability via frequency entrainment, resulting in an anticorrelation between annual cycle strength and ENSO amplitude on precessional time scales.

scholarly journals Freshwater Flux and Ocean Chlorophyll Produce Nonlinear Feedbacks in the Tropical Pacific

2019 ◽  
Vol 32 (7) ◽  
pp. 2037-2055 ◽  
Author(s):  
Rong-Hua Zhang ◽  
Feng Tian ◽  
Antonio J. Busalacchi ◽  
Xiujun Wang
Keyword(s):  
Climate Models ◽  
Southern Oscillation ◽  
Vertical Mixing ◽  
Coupled Model ◽  
Underlying Mechanism ◽  
Tropical Pacific ◽  
Freshwater Flux ◽  
Related Individual ◽  
Nonlinear Modulation ◽  
The Tropical Pacific

Abstract Various forcing and feedback processes coexist in the tropical Pacific, which can modulate El Niño–Southern Oscillation (ENSO). In particular, large covariabilities in chlorophyll (Chl) and freshwater flux (FWF) at the sea surface are observed during ENSO cycles, acting to execute feedbacks on ENSO through the related ocean-biology-induced heating (OBH) and FWF forcing, respectively. At present, the related effects and underlying mechanism are strongly model dependent and are still not well understood. Here, a new hybrid coupled model (HCM), developed to represent interactions between the atmosphere and ocean physics–biology (AOPB) in the tropical Pacific, is used to examine the extent to which ENSO can be modulated by interannually covarying anomalies of FWF and Chl. HCM AOPB–based sensitivity experiments indicate that individually the FWF forcing tends to amplify ENSO via its influence on the stratification and vertical mixing in the upper ocean, whereas the OBH feedback tends to damp it. While the FWF- and OBH-related individual effects tend to counteract each other, their combined effects give rise to unexpected situations. For example, an increase in the FWF forcing intensity actually acts to decrease the ENSO amplitude when the OBH feedback effects coexist at a certain intensity. The nonlinear modulation of the ENSO amplitude can happen when the FWF-related amplifying effects on ENSO are compensated for by OBH-related damping effects. The results offer insight into modulating effects on ENSO, which are evident in nature and different climate models.

Successive Modulation of ENSO to the Future Greenhouse Warming

2008 ◽  
Vol 21 (1) ◽  
pp. 3-21 ◽  
Author(s):  
Soon-Il An ◽  
Jong-Seong Kug ◽  
Yoo-Geun Ham ◽  
In-Sik Kang
Keyword(s):  
General Circulation ◽  
Southern Oscillation ◽  
General Circulation Models ◽  
Tropical Pacific ◽  
Greenhouse Warming ◽  
Delayed Response ◽  
Subsurface Temperature ◽  
Climate System Model ◽  
The Mean ◽  
The Tropical Pacific

Abstract The multidecadal modulation of the El Niño–Southern Oscillation (ENSO) due to greenhouse warming has been analyzed herein by means of diagnostics of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) coupled general circulation models (CGCMs) and the eigenanalysis of a simplified version of an intermediate ENSO model. The response of the global-mean troposphere temperature to increasing greenhouse gases is more likely linear, while the amplitude and period of ENSO fluctuates in a multidecadal time scale. The climate system model outputs suggest that the multidecadal modulation of ENSO is related to the delayed response of the subsurface temperature in the tropical Pacific compared to the response time of the sea surface temperature (SST), which would lead a modulation of the vertical temperature gradient. Furthermore, an eigenanalysis considering only two parameters, the changes in the zonal contrast of the mean background SST and the changes in the vertical contrast between the mean surface and subsurface temperatures in the tropical Pacific, exhibits a good agreement with the CGCM outputs in terms of the multidecadal modulations of the ENSO amplitude and period. In particular, the change in the vertical contrast, that is, change in difference between the subsurface temperature and SST, turns out to be more influential on the ENSO modulation than changes in the mean SST itself.

scholarly journals Skilful predictions of the Asian summer monsoon one year ahead

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Yuhei Takaya ◽  
Yu Kosaka ◽  
Masahiro Watanabe ◽  
Shuhei Maeda
Keyword(s):  
Long Range ◽  
Summer Monsoon ◽  
Southern Oscillation ◽  
Asian Summer Monsoon ◽  
Boreal Summer ◽  
Climate Modelling ◽  
Large Ensemble ◽  
Asian Society ◽  
Asian Summer ◽  
One Year

AbstractThe interannual variability of the Asian summer monsoon has significant impacts on Asian society. Advances in climate modelling have enabled us to make useful predictions of the seasonal Asian summer monsoon up to approximately half a year ahead, but long-range predictions remain challenging. Here, using a 52-member large ensemble hindcast experiment spanning 1980–2016, we show that a state-of-the-art climate model can predict the Asian summer monsoon and associated summer tropical cyclone activity more than one year ahead. The key to this long-range prediction is successfully simulating El Niño-Southern Oscillation evolution and realistically representing the subsequent atmosphere–ocean response in the Indian Ocean–western North Pacific in the second boreal summer of the prediction. A large ensemble size is also important for achieving a useful prediction skill, with a margin for further improvement by an even larger ensemble.

Middle to late-Holocene decreased fluvial aggradation and widespread peat initiation in the Ishikari lowland (northern Japan)

The Holocene ◽  
2016 ◽  
Vol 26 (12) ◽  
pp. 1924-1938 ◽  
Author(s):  
Yuji Ishii ◽  
Kazuaki Hori ◽  
Arata Momohara ◽  
Toshimichi Nakanishi ◽  
Wan Hong
Keyword(s):  
Sea Level ◽  
Late Holocene ◽  
Climate Changes ◽  
Asian Summer Monsoon ◽  
Sedimentary Environment ◽  
New Approach ◽  
Sea Level Fluctuations ◽  
Asian Summer ◽  
Fluvial Sedimentation ◽  
Northern Japan

This study investigated the influence of sea-level and climate changes on the decreased fluvial aggradation and subsequent widespread peat initiation in the middle to late-Holocene in the Ishikari lowland, which is a coastal floodplain formed in response to the postglacial sea-level change. By introducing a new approach to separately evaluate the rates of organic and clastic sediment input, we demonstrated that the peat began to form when the fluvial sedimentation rate was significantly decreased (less than 0.6 mm/yr), while plant macrofossil analysis suggested that lowering of water level is also important to the peat initiation. Such changes in sedimentary environment may be associated with the abrupt abandonment of crevasse splays. The concentrated ages of the peat initiation around 5600–5000, 4600–4300, and 4100–3600 cal. BP suggest that an allogenic control promoted the abandonment of crevasse splays, and different onset ages can be explained by different fluvial responses of the Ishikari River and its tributaries. The abandonment of crevasse splays could result from sea-level fall or decreased precipitation. While submillennial sea-level fluctuations coincident with the peat initiation have not been reported in coastal lowlands of Japan, the close comparison of the onset ages and decreased precipitation recorded in a stalagmite from China, which represents the strength of the East Asian summer monsoon (EASM), suggests that decrease in precipitation led to the abandonment of crevasse splays. Our results may indicate that similar fluvial responses might be common in other coastal floodplains affected by the EASM.

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